The present invention relates to a refrigeration process for liquefying a gas. More particularly, the present invention relates to a process for the liquefaction of natural gas which is more energy efficient than prior methods and, thus, more economical.
Numerous reasons exist for the liquefaction of gases and particularly of natural gas. The primary reason for the liquefaction of natural gas is that the liquefaction reduces the volume of a gas by a factor of about 1/600, thereby making it possible to store and transport the liquefied gas in containers of more economical and practical design.
For example, when gas is transported by pipeline from the source of supply to a distant market, it is desirable to operate under a substantially constant high load factor. Often the capacity will exceed demand while at other times the demand may exceed the capacity of the pipeline. In order to shave off the peaks where demand would exceed supply, it is desirable to store the gas when the supply exceeds demand, thereby peaks in demand can be met from material in storage. For this purpose it is desirable to provide for the storage of gas in a liquefied state and to vaporize the liquid as demand requires.
Liquefaction of natural gas is of even greater importance in making possible the transport of gas from a source of plentiful supply to a distant market, particularly when the source of supply cannot be directly joined with the market by pipeline. This is particularly true where transport must be made by ocean going vessels. Ship transportation in the gaseous state is uneconomical unless the gaseous material is highly compressed, and even then the transportation system would not be economical because it is impractical to provide containers of suitable strength and capacity.
In order to store and transport natural gas, the reduction of the natural gas to a liquefied state requires cooling to a temperature of about -240.degree. F. to -260.degree. F. at atmospheric pressure.
Numerous systems exist in the prior art for the liquefaction of natural gas or the like, in which the gas is liquefied by passing it sequentially through a plurality of cooling stages to cool the gas to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, and methane which are expanded in a closed refrigeration cycle. Additionally, the natural gas is expanded to atmospheric pressure by passing the liquefied gas through one or more expansion stages. During the course of the expansion, the gas is further cooled to a suitable storage or transport temperature and its pressure reduced to atmospheric pressure. In this expansion to atmospheric pressure significant volumes of the natural gas are flashed. The flashed vapors from the expansion stages are generally collected and recycled for liquefaction or else burned to generate power for the liquid natural gas manufacturing facility.
These prior liquefaction processes have typically carried out the depressurization of the natural gas feed by Joule-Thompson expansion, or constant enthalpy expansion, and has resulted in a reduced pressure and temperature for the gas feed system. Such expansions are uneconomical and wasteful because the gas expands without doing any useful work. Therefore, it would be desirable to develop a liquefaction process which extracts useful work from expansion of the gas feed stream. Moreover, it would be desirable to develop a more efficient arrangement for a process for the liquefaction of natural gas.